The present invention generally relates to vehicle brakes, and more particularly relates to a system for adjusting brake pressure.
Apart from a primary pressure source for hydraulic fluid (which is e.g. the brake cylinder/tandem master cylinder), modem vehicle brake systems include one or more hydraulic pumps by which hydraulic fluid that may also be pressurized can be supplied for certain purposes. These pumps may be pumps mounted on the valve block and actuated by way of an electric motor and an eccentric. The purpose of these pumps is to build up the pressure of the hydraulic brake fluid for specific controlling (e.g. traction slip control) or regulating purposes in case the brake pressure which originates from the main pressure source is not sufficient to achieve the objective of the control. Generally, for example, during traction slip control, the driver does not brake at all and accordingly the primary pressure source does not pressurize the hydraulic brake lines. Nevertheless, active pressure increase, especially on the brakes of the driven wheels, is often desirable for traction slip control. This pressure increase can be generated by the pump mentioned hereinabove.
Up to date, a pump of this type has been switched on and off electrically. The delivery rate and, thus, indirectly the brake pressure is adjusted in two different ways. On the one hand, the delivery volume of the pump can be limited on the suction side, for example, such that a valve between the fluid reservoir and the pump is opened and closed in conformity with defined criteria. The disadvantage is that the pump produces a vacuum on the suction side, which may cause leakage problems, e.g. with respect to low-pressure accumulators connected to the suction side of the pump. Another possibility of adjusting the brake pressure includes the actuation of the inlet valves of a wheel brake for pressure adjustment. This possibility is, however, disadvantageous because the pump operates in opposition to the closed valves. Noises develop therefrom which become audible to the driver and leave the impression that there is something malfunctioning with the vehicle brake system.
Further, the prior art method of installation of inlet valves requires non-return valves to avoid the possibility of sticking inlet valves. The case occurs relatively frequently that, based on an ABS brake operation, the brake pressure is decreased by the driver, with the inlet valve of the wheel brake being closed. On the other hand, the case occurs that a higher pressure prevails at the inlet valve on the outlet side (brake side) than on the inlet side (pressure source side). To avoid a stuck condition of the valves in this situation, there is provision of non-return valves which permit pressure reduction past the inlet valve in a downstream direction. The arrangement of these non-return valves complicates the design of a valve block still further.
Besides, DE 44 405 17 A1 discloses a method and a device for actuation of a return pump of a brake system wherein brake pressure from the return pump is controllable in dependence on a controller which is adapted to influence the driving speed of a vehicle. The actuation for brake pressure adjustment is effected according to the wheel speeds. An object of the present invention is to provide an improved method and device for adjusting brake pressure and for opening an inlet valve.
For brake pressure adjustment, the pump is actuated electrically in such a fashion that it is operated by degrees, that means, that operating conditions, delivery rates, rotational speeds between on and off can also be adjusted. For example, the electric motor of the pump can receive a pulse-width-modulated signal so that the motor""s rotational speed and, thus, the delivery rate, can be adjusted continuously or quasi continuously between 0 and 100% of the maximum rate of delivery. This eliminates the need for the strategies for brake pressure adjustment (suction-side limitation or limitation by the inlet valves upstream of the wheel brake) known from prior art so that the related disadvantages, too, are overcome. The pump itself will then be the decisive control element for the adjustment of the brake pressure, and the inlet valve and the valve on the suction side of the pump can remain open to full extent.
According to an advantageous design, the pump receives pulse-width-modulated minimum pulses between 5 and 20 milliseconds (ms), the minimum duration of which is so defined by the clock generator circuit or delay circuit that high xe2x80x98disconnecting currentsxe2x80x99 are avoided to prevent thermal load on the circuit. In this arrangement, the pump is actuated by degrees according to the determined brake pressure or nominal pressure depending on the pump""s generator voltage and/or its delivery rate and/or its rotational speed. This provision permits adjusting the determined brake pressure or nominal pressure, which is preferably calculated in a pressure model, initially from the delivery volume of the pump run-on, even when the pump is switched off, before the pump is actuated again by degrees on command of minimum pulses. For the new actuation of the pump by degrees, at least one of the following conditions must be satisfied:
a) the generator voltage is below a predetermined limit value and/or
b) the delivery rate of the pump is below a predetermined limit value and/or
c) the rotational speed of the pump is below a predetermined limit value.
All limit values lie in bands which appear after deactivation of the pump, that means, below the actuation quantities of the minimum pulses. Preferably, pressure buildup is no longer possible below the limit values when the pump is switched off. Because the pump is actuated by minimum pulses and the pressure which is produced by the pump in its run-on, when the pump is switched off, is used for brake pressure increase, and the pump is actuated again only by degrees according to the brake pressure or nominal pressure determined in dependence on its generator voltage, and/or delivery rate, and/or rotational speed, the emission of noise is reduced. This is due to the fact that a lower system pressure level occurs and noises of overflow caused in the overflow valve are reduced or totally avoided. The hydraulic components, especially the pump and the separating valve, are exposed to reduced load. In addition, control quality is enhanced by the easier and more precise approach to the pressure required.
The difference in pressure between the inlet side and the outlet side of the inlet valve can be detected to open a closed inlet valve. When this difference in pressure exceeds a critical value, the mentioned hydraulic pump can be switched on briefly in order to cause pressure buildup at the inlet valve of the inlet side. This decreases the difference in pressure, and the valve returns to its open (normally deenergized) position. The need for the four non-return valves which respectively by-pass one of the inlet valves is thereby obviated.